The present invention relates to the conditioning of a recycle stream for use in ozone generation processes. More particularly, the present invention relates to the elimination of contaminants from an ozone recycle stream resulting from ozone bleaching of wood pulp.
In general, techniques for generation of ozone are well known to those skilled in the art. While a great variety of methods are known, only two are of significant commercial importance. These methods are ultra-violet radiation of air or oxygen, and corona or electrical discharge in air or oxygen. The ultra-violet radiation method is capable of producing only low concentrations of ozone and is thus generally limited to food preparation processes. The relatively high concentrations of ozone required for processes such as waste treatment or pulp bleaching necessitate the use of the corona or electrical discharge method.
Due to impurities in air, ozone generation using air is limited to concentrations of about 4% by weight. Use of pure oxygen allows the concentration to be increased; however, pure oxygen can be an expensive raw material. It is therefore desirable to recycle the oxygen containing gas discharged after the ozone is used for its intended purpose. When this gas is recycled, various contaminants must be removed from the recycle stream to maintain a minimum efficiency in the ozone generation process.
A number of different techniques are known in the art for removing contaminants, including carbon dioxide, from an ozone generation recycle stream. For example, U.S. Pat. No. 3,151,943 discloses a method for purifying exit oxygen in an ozone generation system. This reference is typical of the prior art references in that it recognizes that contaminants affect the efficiency of ozone generation, but does not discuss a specific relationship between contaminant level and generation efficiency. Generally contaminants were looked upon as diluents which displace oxygen.
U.S. Pat. Nos. 3,963,625, 3,748,262 and 4,430,306 disclose the use of molecular sieve materials or adsorbent materials such as zeolite and aluminosilicate to remove contaminants in an ozone generation recycle system. U.S. Pat. Nos. 4,287,130 and 4,399,292 each disclose counter-current scrubbing with alkaline materials such as sodium hydroxide to remove organic compounds from the recycle stream. The '130 patent also discusses scrubbing with bicarbonate solutions to remove carbon dioxide, as well as tapping out inorganic components of the exhaust gas. U.S. Pat. No. 4,132,637 discloses an ozone waste treatment system wherein only a portion of the "off gas" is recycled to the ozone generator in order to reduce the amount of contaminants in the feed gas.
In addition to the above references, U.S. Pat. Nos. 2,700,648, 3,421,999, and 3,715,430, each disclose the removal of carbon dioxide in an ozone generation feed gas, although not utilizing a recycle system. The '430 and '999 patents use activated charcoal filters. The '648 patent contemplates the use of adsorbers or scrubbers which utilize sodium hydroxide or soda lime.
The references discussed above all describe methods and apparatus for removing various contaminants from a recycle stream or eliminating carbon dioxide from an ozone generation feed gas. However, none of these references discuss specific levels of contaminants or contaminant removal. Additionally, none of the above references disclose ozone generating and recycle gas conditioning under conditions associated with ozone bleaching of wood pulp. U.S. Pat. No. 4,279,694 shows a block diagram of an ozone recycle system in a pulp bleaching process. However, no details of operation are disclosed and, like the references discussed above, required levels of contaminant removal are not discussed. Ozone pulp bleaching can produce high levels of carbon dioxide; levels which apparently were not appreciated in the various systems disclosed in the prior art.
Prior art predicted ozone generating efficiency does not show a particular concern with respect to carbon dioxide levels over all operational ranges. As with other contaminants, carbon dioxide was considered primarily a diluent with any detrimental effects on ozone generation caused simply by a reduction in the amount of oxygen present. Cromwell and Manely, Effect of Gaseous Diluents on Energy Yield of Ozone Generation from Oxygen, Ozone Chemistry and Technology, Advances in Chemistry Series No. 21, pp. 309-10, (American Chemical Society, March, 1959) state that little loss in energy yield (ozone produced per unit energy applied) is expected for carbon dioxide concentrations in the feed gas up to 10 wt %. This has become generally accepted in the art.
Ozone generation efficiency depends on a variety of factors other than carbon dioxide content and overall oxygen purity. The general effect of various factors is discussed in Nebel, Ozone, "Encyclopedia of Chemical Technology", vol. 16, pp. 693-96 (3d ed., John Wiley & Sons 1981). Of the various factors discussed, generator size, power density, flow rate most directly impact on the cost of generation. Thus, it is possible to maintain apparent generation efficiency at high carbon dioxide contents. However, while increasing the generator size, reducing power density or reducing the actual flow rate maintains an apparent efficiency, capital costs are increased or productivity is decreased. As a result, no overall increase in cost efficiency of the ozone generation plant and recycle system is actually achieved.